Field of the Invention
The present invention relates to an oriented capacitor film having a high voltage resistance, an extremely thin film thickness, and an excellent processing suitability for winding elements or the like, and also relates to a cast raw sheet available for producing the capacitor film.
Description of the Related Art
A biaxially oriented polypropylene film has been widely used as a raw film for industrial application such as a packaging film. Moreover, the biaxially oriented polypropylene film has also been used as a dielectric film for a capacitor, because the biaxially oriented polypropylene film has a particularly high humidity resistance in addition to excellent electronic properties such as a high voltage resistance and low dielectric loss characteristics, these properties being advantageous in capacitor applications.
When the biaxially oriented film is used as a capacitor film, the surface thereof is required to be finely roughened in moderation so as to improve the following properties: processing suitability for winding elements at the time of producing a capacitor; sliding properties of the film at the time of processing the film; and oil impregnation properties when the film is applied to an oil-impregnated capacitor (see Patent Document 1: Japanese Unexamined Patent Application, First Publication No. S 51-63500, pages 2-4).
As methods for finely roughening the surface, various methods such as mechanical methods such as an embossing method or sandramie method, chemical methods such as a chemical etching method using solvents, methods of drawing sheets in which dissimilar polymers are blended or copolymerized, or sheets in which a β-form is formed, have been conventionally proposed (see, for example, Patent Document 1).
According to “Introduction to high polymer under polarizing microscope” attributed to Hiroshi Kuriya, published by AGNE GIJYUTSU CENTER, page 131, 2001, polypropylene resins usually have crystal polymorphism such as α-form, β-form, or the like.
The β-form has a low density, a low melting point, and other different physical properties, in comparison to the α-form. When a molten polypropylene resin is crystallized within a particular temperature range, the β-form is generated. When this β-form is drawn at the vicinity of the melting point thereof, a spherulite of β-form transfers to a spherulite of α-form with a density different from that of β-form. The density-difference between these crystal forms generates fine unevenness on the film surface. When the surface is roughened by this method, impurities such as additives are not required to be added to the resin. Accordingly, this method is advantageous in forming extremely fine unevenness without deteriorating electric characteristics thereof.
When the surface is roughened by the β-form, how the generation of the β-form is controlled at the time of producing a sheet is technically important. As for the generation of the β-form, Patent Documents 2 to 4 (Japanese Unexamined Patent Application, First Publication No. 2004-2655, pages 3 to 7; Japanese Unexamined Patent Application, First Publication No. 2004-175932, pages 4 to 8; Japanese Unexamined Patent Application, First Publication No. 2005-89683, pages 5 to 7), for example, disclose that a sheet with a high β-form proportion can be produced from a polypropylene resin obtained by polymerization using a particular catalyst, the polypropylene resin having a certain range of melt flow rate, molecular weight, and molecular weight distribution.
In addition, as for the preparation of an oriented polypropylene film with a roughened surface, Patent Document 5 (Japanese Patent Publication No. 3,508,515, pages 2 to 3) discloses a manufacturing technique in which a polypropylene raw resin with a particular stereoregularity is used to control the content of β-form in a cast raw sheet to a particular content or more.
However, such a surface roughening generally has the disadvantage of inviting a degradation of voltage resistance, although the surface roughening is essential to enhance the processing suitability. In contrast, demand for an industrial capacitor has increased, and a request for a capacitor with a higher voltage resistance is very strong in the market. In addition to this request, further enhancement of the electric capacity is also requested.
The voltage resistance can be enhanced by increasing the smoothness of the surface, or making a polypropylene resin have high stereoregularity and crystallinity in accordance with Patent Document 6 (Japanese Unexamined Patent Application, First Publication No. H 8-294962, pages 2 to 3), for example.
However, high crystallinity causes deterioration of the extensibility and causes the film to be easily torn at the time of drawing, which is unfavorable for manufacturing the film.
On the other hand, a dielectric film is required to be thin so as to enhance the electric capacity of a capacitor while maintaining a constant volume. In order to produce such a thin film, the extensibility of a resin and a cast raw sheet is required to be enhanced. However, the enhancement of the extensibility is generally incompatible with the enhancement of the voltage resistance, that is, crystallinity enhancement, as described above.
Thus, a capacitor film and a raw sheet thereof satisfying the following three characteristics required by the market: (1) processing suitability for producing a capacitor (roughened surface), (2) high voltage resistance (smoothed surface, high crystallinity), and (3) high electric capacity (high extensibility for producing an extremely thin film) have not been provided till now.